Constraining SMEFT BSM scenarios with EWPO and \(\Delta _{\textrm{CKM}}\)

Precision observables are well known for constraining most of the beyond standard model (BSM) scenarios tightly, among which the electroweak precision observables (EWPO) play a significant role. Recent measurement of the W boson mass (\(M_W\)) at CDF-II has put a renewed focus on the EWPO and its ensuing constraints. In this study, we look at the scenarios where these BSM effects are encoded primarily in ten dimension-6 standard model effective field theory (SMEFT) operators (in the Warsaw basis) which contribute to the EWPO at tree-level. After presenting the fitting procedure and testing it on the S, T and V parameters, we go on to fit various BSM scenarios consisting of different well-motivated subsets of the 10 operators of our interest. To constrain these scenarios further, we augment the EWPO with \(\Delta _{\textrm{CKM}}\) measurement using 1-loop matching of the low-energy effective field theory to SMEFT operators at the Z-pole. We show that the inclusion of the \(\Delta _{\textrm{CKM}}\) constraint indeed results in stronger bounds on our Wilson coefficients. One of our operator subsets caters to the models of vector-like leptons, where we find out that such a minimal extension is in tension with the forward–backward asymmetry in the b-sector (\(A_b^{\textrm{FB}}\)) and \(M_W\). Finally, we also include the LEP-II observables pertaining to the WW production, thereby lifting the two associated blind directions, helping us present the results for the full fit.